1. Technical Field of the Invention
The present invention relates generally to relates generally to an automotive automatic transmission controller equipped with a feature to increase an engine power for minimizing a shift shock occurring during downshift control executed according to a driver's deceleration intention to develop the engine braking.
2. Background Art
Most modern automotive automatic transmissions are designed to switch between hydraulic control lines to establish engagement and disengagement of frictional elements such as clutches and brakes selectively for achieving a desired one of speeds. When the accelerator of the vehicle is released fully during running on a downhill, but the engine braking is insufficient, the driver of the vehicle usually turns off the overdrive switch or moves the shift lever from the D to S or L range to make a downshift for increasing the engine braking.
Such a downshift to increase the engine braking with the accelerator released will result in an increased gear ratio of the automatic transmission, thus elevating the speed of the engine. In such a running mode of the vehicle to set up the engine braking, the throttle valve is usually closed, so that the torque is transmitted from an output shaft of the transmission to the engine through some of the frictional elements working to establish an after-downshift speed to increase the speed thereof. This may cause the time required for the downshift to be increased, thus giving rise to a delay in the engine braking or inertia torque arising from the increased speed of the engine to appear as braking torque, which results in an instantaneous increase in the engine braking, leading to a large-scale shift shock. A rapid rise in torque transmitted through the frictional elements of the automatic transmission under hydraulic control will result in a quick rise in speed of the engine, so that the downshifting time decreases, but however, the braking torque increases excessively, thus resulting in a more increase in the shift shock.
Japanese Patent No. 2924463 teaches a transmission control system for avoiding the above problems. This system includes a function to increase the power of the engine temporarily upon a downshift to the speed of the automatic transmission which creates the engine braking with the accelerator released.
Japanese patent First Publication No. 2002-310277 discloses a downshift control system which is designed not to increase the power of the engine upon the downshift, but to perform down shift control in a sequence of three stages. In the initial stage, the system commences quick loading control to charge or load the working fluid to one(s) of frictional elements of an automatic transmission (will be referred to as an on-coming clutch below) which is to be engaged to achieve the downshift until just before the on-coming clutch is engaged. In the second stage, the system decreases and keeps the hydraulic pressure acting on the on-coming clutch at a standby level to develop the action of the downshift. In the final stage, the system increases the hydraulic pressure acting on the on-coming clutch to complete the downshift at a time when the speed of an input shaft of a transmission gearbox rises and reaches one corresponding to the downshifted speed.
When the amount of working fluid loaded to the on-coming clutch during the quick loading control becomes excessive, it will cause the on-coming clutch to be engaged before the speed of the input shaft of the transmission gearbox reaches the above value, thereby resulting in an early downshift to contribute to an uncomfortable shift shock.
Conversely, when the amount of working fluid loaded to the on-coming clutch becomes lacking, it will result in a time lag between a rise in the speed of the input shaft of the transmission gearbox up to the one corresponding to the downshifted speed and actual engagement of the on-coming, which leads to the delay of the downshift. This results in an undesirable quick rise in the speed of the input shaft of the transmission gearbox to induce the shift shock.
In order to avoid the above problems, the downshift control system, as taught in the above publication, works to learn the overloading or underloading of the working fluid to the on-coming clutch under the quick loading control using characteristics of a rise in the speed of the input shaft of the transmission gearbox to correct a time duration for which the quick loading control is to be executed or a hydraulic pressure command to determine the hydraulic pressure to be applied to the on-coming clutch upon a following downshift request.
Japanese Patent First Publication No. 6-235451 teaches a downshift control system which is like the above latter publication, designed not to increase the power of the engine upon the downshift, but to learn the time duration for which the quick loading control is to be executed or the hydraulic pressure command to be used upon a following downshift request so as to bring the time the speed of the input shaft of the transmission gearbox into agreement with the time of completion of the quick loading control in order to correct the excess or lack of the working fluid loaded to the on-coming clutch under the quick loading control.
For the purpose of eliminating the shift shock or time lag of the downshift, the engine power increasing control, as taught in the first publication, and learning correction in the quick loading control, as taught in the second or third publication, may be combined to control the downshift of the automatic transmission.
Such a combination, however, results in a difficulty in specifying whether a rise in the speed of the input shaft speed of the transmission gearbox after the quick loading control has arisen from the engine power increasing control or the completion of loading of the working fluid to the on-coming clutch, which may lead to an error in learning and correcting the excess or lack of the working fluid loaded to the on-coming clutch.